IR Spectroscopy Flashcards
What colour is IR light?
RED
Range of IR we are interested in?
2500- 5000nm
What is wave number?
V = 1/ wavelength(cm)
Relation between energy and V ?
Higher the V, more the energy
Do molecules excite after absorbing IR?
NO- IR energy too low- molecules remain in ground stage after absorbing
When is a molecule IR active?
Must be a change in dipole moment as a result of the vibration that occurs when IR is absorbed
Why does it show a broad bend instead of sharp peaks?
Each vibration is associated with several rotational levels- so broad bend produced rather than sharp peaks
IR spectra x axis and y axis?
Y- %T
X- V (4000- 650cm-1)
Absorption of IR is shown through PEAKS- True or False?
FLASE- shown by troughs
Do bonds vibrate all the time?
YES
Do molecules bend and stretch at certain frequencies?
YES
Frequency at which a given bond vibrates is constant - T or F?
TRUE
In mid IR region- molecule can go from V1 to V2 directly - T or F?
FALSE- can only go from Vo —> V1, V1–> V2
Two different molecules can have the same IR spec- T/F
FALSE- no two molecules can have same IR spec- that’s why used for comparison and ID
What are the two types of vibrations?
Stretching
Bending
Two types of stretching?
Symmetrical
Asymmetrical
Types of in- plane BENDING?
Scissoring
Rocking
Types of out of plane BENDING?
Wagging
Twisting
Asymmetric occur at higher f than symmetric - T/F
TRUE- asymmetric requires more energy
Are all vibrations IR active?
NO- only those that result in a net change of polarity - dipole moment
Correlation between vibration and abs?
Vibrations that have bigger dipole moment—> stronger absorption
Are symmetrical bonds IR active?
NO- no dipole moment- no IR peaks
How to find out if a molecule has a dipole moment?
If centre of the +be charge is different to centre of -ve charge
More electronegativity difference = bigger dipole moment intensity?
TRUE
Does bending require less energy than stretching?
YES- takes more energy to excite stretching vibration than bending
Higher V- shorter wavelength- higher F
To absorb IR, a molecule MUST…?
- have a dipole change during vibration
- have molecular asymmetry by stretching and or bending
Absorbance of IR of a bond depends on which 3 things?
1- mass of atoms connected to bond- if heavier, harder to stretch
2- dipole moment
3- strength of bond- stronger bond need more energy to vibrate- harder to stretch
